Honeycomb core composite sandwich panels are commonly employed in the aerospace industry as they are extremely light weight yet have high strength. Honeycomb core composite sandwich panels are comprised of upper and lower fiber-reinforced composite laminates separated and stabilized by a core of honeycomb. Due to the high bending stiffness and compressive strength properties (e.g., the honeycomb serves as a shear web and separates the upper and lower composite laminates from the bending neutral axis) the panels are particularly useful for aircraft fuselage and cargo door structures.
Typically, the honeycomb core panels are fabricated in an open-faced mold while vacuum bagged and cured under pressure and temperature in an autoclave. Alternatively, these panels are often fabricated in a matched mold and/or press. In either case the lower laminates, honeycomb core and upper laminates are sequentially stacked in the mold. Film and/or paste adhesives are used between the upper and lower laminates and the honeycomb core. The upper and lower laminates may be uncured preimpregnated composite material, precured composite material or sheet metal skins. When laminated in an open-faced mold, the completed laminate is vacuum bagged and autoclave cured. Laminates in matched molds are cured in a press.
Typically, edge closeout laminates are interleaved between layers of the upper and lower laminates. However, this creates problems because the edge closeout does not receive adequate pressure to achieve a sufficient cure of the preimpregnated material. One solution has been to fill the core periphery with a syntactic filler or foaming adhesive which imparts rigidity to the panel edge. However, this solution adds substantial weight. In addition, the foam or filler creates an imperfect edge which subsequently impacts the quality and strength of closeout laminates since the foam or filler material is not machineable. Finally, if the compaction pressure applied to the panel edge during the cure cycle is not uniform, the edge laminates may become resin rich or resin starved.
U.S. Pat. Nos. 4,335,174 and 4,076,877 disclose two honeycomb core composite sandwich panel edge configurations. U.S. Pat. No. 4,335,174 discloses a foam adhesive in combination with a tape adhesive that is in conformance with the shape of the honeycomb core. The adhesives are disposed external to the upper and lower sandwich composite panels. U.S. Pat. No. 4,076,877 discloses an end closure for a honeycomb panel which is a two-layer or a laminate plastic ribbon. A first uncured layer is disposed adjacent to the honeycomb. A second precured layer is disposed external to the first layer and external to the composite panels that sandwich the honeycomb panel.
Thus, although there have been a variety of configurations for the edge of honeycomb core composite sandwich panels, there is still a continual search for lightweight, high strength, compaction resistant, composite panel edge configurations.